1) Field of the Invention
The invention pertains to the field of polyester synthesis starting from a hydroxyalkyl dicarboxylic acid ester monomer and/or oligomer mixtures and polymerizing these to form a polyester. In particular the present invention is a polyesterification process preferably employing substantially monohydroxyalkyl ester monomer such as monohydroxyethyl terephthalate, with preferably little or no bishydroxyalkyl ester monomer.
The invention also pertains to the field of polyester synthesis starting from traditional raw materials of dicarboxylic acid and diol, reacting these raw materials to form hydroxyalkyl dicarboxylic acid ester monomer and/or oligomer mixtures (primarily monohydroxyalkyl ester monomer with little or no bishydroxyalkyl ester monomer) and polyesterifying these to form a polyester. Both this reaction process and the previous mentioned reaction process occur in a pipe or tubular reactor at superatmospheric pressure wherein turbannular flow occurs.
2) Prior Art
Polyester manufacture is globally practiced, and a variety of methods are taught. The direct esterification of a dicarboxylic acid with a diol, for example terephthalic acid (TA) and ethylene glycol (EG), forms a reactive monomeric material releasing water. The monomeric material is known to contain the species monohydroxyethyl terephthalate, bishydroxyethyl terephthalate, and longer-chain length oligomers of the same structural type having an average degree of polymerization (DP) of from 1 to 6. The DP is increased further by melt polymerization of the monomeric material under vacuum conditions, a process referred to as polycondensation.
A polymerization process is described in U.S. Pat. No. 3,480,587 to Porter where at least part of the polycondensation takes place while the liquid reaction mixture flows in a long, narrow tube with an inert gas such as nitrogen in a two phase flow regime called turbannular flow. The reaction mixture entering the tube has an average DP of from 27 to 40, and the product issuing from the tube has an intrinsic viscosity ratio between 1.7 and 2.0 (measured as a 1% solution in orthochlorophenol at standard conditions) corresponding to 65-100 DP. Porter teaches that in a pressure process using an inert gas "the ratio of the cross-sectional area of the tube divided by the length of the wetted perimeter should be less than 2.5 cm." To maintain this requirement for a tubular reactor, the diameter of the reactor must not exceed four inches.
As taught by Porter, a high weight ratio of gas to polymer is required for polymerization to proceed. To achieve a high weight ratio in the reactor tube high gas velocities are required. If a reaction material has a DP of 20 or less, particularly 10 or less, extremely high and impractical gas velocities would be required to achieve a practical increase in DP under turbannular flow. As taught by Porter, then, it would be economically infeasible to manufacture polyester from lower DP reaction materials in a tubular reaction zone under turbannular flow.
U.S. Pat. No. 5,434,239 to Bhatia discloses an atmospheric-pressure process for the continuous production of polyester via a melt of bishydroxyethyl terephthalate (BHET) or its low molecular weight oligomer. BHET is intimately contacted with nitrogen gas which flows counter current to the melt, to facilitate polymerization and removal of the volatile reaction by-products. A degree of polymerization of 15 to about 30 is achieved in the prepolymer stage. The DP is further increased to about 50-100 DP in the finishing stage with a counter current flow of nitrogen. Polymerization occurs in both stages without resorting to a vacuum. Bhatia teaches that the nitrogen velocity is critical to the success of the process, and that the nitrogen velocity should be between 0.2 and 5.0 ft/sec. The counter current flow process Bhatia describes does not include operation in the turbannular flow regime, which by definition is a co-current process.
WO 96/223 18 to Iwasyk et al. discloses a multi-stage process for producing polyester oligomers without vacuum. In the first stage a polyol is added to esterified oligomer feed material in a pipeline reactor. Inert gas is injected into the oligomeric product at the end of the first stage, to carry the oligomeric material into the following stages. The inert gas also serves to provide a pressure drop along the tubular reactor, which aids in the removal of volatile reaction by-products. The amount of inert gas used in the process of Iwasyk et al. is less than two pounds per pound of oligomer, and the flow regime in the tubular reactors is not turbannular. It is taught that a prepolymer with a DP of from 2 to 40 and a carboxyl to hydroxyl end group balance between about 1:2 and 1:8 is produced at the exit of the final stage of the pipeline reactor. It would be desirable to efficiently and economically obtain a relatively higher DP polymer (40 or more) with a carboxyl to hydroxyl end group ratio that avoids premature arresting of the polymerization.
WO 96/39456 to DeSimone discloses a transesterification-polycondensation process for polyester using carbon dioxide as the polymerization medium. The polycondensation of BHET conducted in an autoclave with flowing supercritical carbon dioxide is exemplified. Ethylene glycol is liberated for every step growth in DP. A 10-50 mole percent excess of glycol is recommended. The highest DP achieved in the examples is 33. As exemplified by DeSimone, excess diol is removed by flowing carbon dioxide or by including a surfactant with carbon dioxide capable of scavenging the condensate within the reactor without removing carbon dioxide. Higher DP increases would be desirable without requiring purification of the diol to remove the surfactant. Furthermore, as demonstrated by DeSimone, a DP of about 33 was achieved in a continuous flow process starting with BHET. However, this required a relatively large amount of carbon dioxide, i.e., a weight ratio of carbon dioxide to BHET of 43:1.
In general, it is known that dicarboxylic acids and diols react with the removal of water to form polyesters, which under favorable conditions will increase in polymer chain length. More specifically, with respect to conventional esterification of polyester monomers, the degree of polymerization (DP) obtained is a function of operating pressure for the repeating unit: ##STR1## wherein R is contributed by the diol, which for polyethylene terephthalate (PET) is --CH.sub.2 CH.sub.2 --, and n is the degree of polymerization. The DP of the oligomer is determined by dividing the number-average molecular weight by the molecular weight of the repeating unit, which for PET is 192. In characterizing the mole percentage of components of a reaction material which may contain monomeric and oligomeric components, the number average DP of any oligomers must be determined to find the molecular weight of the oligomers.